Considering it seems to be the most common complaint about the trailer, I think it goes beyond just the people who doubted the casting to begin with, or don't like Eisenberg or would have preferred someone else. I'm sure for some people one of those might be the reason, but not for everyone.
Nothing would have made me more happy than to see him shine in the trailer and I still hope he does in the movie. Lex Luthor is a great character, filled with tons of potential to be great on screen. Unfortunately, I never thought Eisenberg was capable of living up to that potential and after this trailer I can say that he's playing the role pretty much exactly as I expected he would and it's just not what I was hoping for when it comes to one of the greatest villains in Comic books. When you're on the doorstep looking forward to an all new DC Universe, you start to imagine what the characters will be like and how you would like to see them portrayed, I wanted the Joker and Lex to be reoccurring characters in this new DCU, showing up a few times each. I wanted them to headline the villains of the DCU and play huge roles across several movies. So there is no doubt that I wanted anyone who was cast in those roles to succeed. Unfortunately, this is just basically the opposite of what I was hoping for with Lex.
I thought he was pretty bad in the trailer, definitely the worst of anyone shown in the trailer. Of course, that's just me refusing to give him a chance according to some.

I have kind of the opposite view as you, I can't believe how some people watched that trailer and thought he was good. But as always, to each their own and I don't begrudge anyone that thinks he's going to be great or loved his scenes in the clip. I just don't feel the same way.